The invention relates to a photomultiplier tube and particularly to a method for stabilizing the anode sensitivity of the tube.
The loss of anode sensitivity, commonly described as tube fatigue, is a function of anode current level, dynode material and previous tube operating history.
The sensitivity changes that are a direct function of high anode currents imposed for extended lengths of time are believed to be the result of erosion or rearrangement of the alkali material from the dynode surfaces during periods of heavy electron bombardment and the subsequent deposition of the alkali material on the other areas within the photomultiplier tube. Sensitivity losses of this type may be reversed during periods of non-operation when the alkali material may again return to the dynode surfaces. This process of return may be accelerated by heating the photomultiplier tube during periods of non-operation to a temperature within the maximum temperature rating of the tube.
Sensitivity losses for a given operating current usually occur rather rapidly during initial operation and at a much slower rate after the tube has been used for some time. Tubes operated at lower anode current levels, of the order of 10 microamperes or less, experience less fatigue than those operated at higher currents.
Fatigue rates are also affected by the type of dynode material used in the tube. Beryllium-copper or silver-magnesium dynodes are generally more stable at high operating currents than alkali antimonide dynodes. It has been observed that the anode sensitivity for tubes utilizing beryllium-copper dynodes and silver-magnesium dynodes very often increases during the initial hours of operation, after which a gradual decrease in sensitivity takes place.
In many photomultiplier tube applications, particularly where more than one photomultiplier tube is used, the above-described sensitivity changes are objectionable. For example, in gamma camera systems which typically utilize as many as 37 photomultiplier tubes, the problem of anode sensitivity changes is especially troublesome since tube-to-tube variations in sensitivity are difficult to control. In such systems the maximum anode sensitivity changes that can be tolerated are within the range of .+-.10% for each tube.
In photomultiplier tubes such as the RCA 4879A used in some gamma camera systems a plurality of beryllium-copper dynodes such as those described in the copending patent application, Ser. No. 132,659, filed Mar. 21, 1980, by Faulkner et al., assigned to the same assignee as the present invention and incorporated by reference herein, are used. An improved secondary emission surface of the primary dynode in the above-mentioned Faulkner et al. application provides a greater number of secondary electrons for each primary electron incident thereon. The higher electron gain from the first dynode results in a greater flux of electrons incident on subsequent dynodes in the electron multiplier array with a resulting increase in anode current and a correspondingly greater change in anode sensitivity.